Method for improving GPS integrity and detecting multipath interference using inertial navigation sensors and a network of mobile receivers

ABSTRACT

A method for checking the integrity of GPS measurements for a moving vehicle includes determining a first inter-vehicle distance between a first vehicle and a second vehicle based on GPS measurements obtained at both vehicles, independently determining a second inter-vehicle distance based on relative motion of the first vehicle and the second vehicle obtained using INS sensors at both vehicles, and comparing the first and second inter-vehicle distances. The integrity of the GPS measurements are checked if the first and second inter-vehicle distances are nearly equivalent. Methods for error detection and for mapping GPS multipath levels at each point in a vicinity for an entire range of satellite constellations are also described.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division application of prior U.S. applicationSer. No. 10/615,427 filed Jul. 7, 2003, now U.S. Pat. No. 7,110,882which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to vehicle navigation using the globalpositioning system (GPS) and more particularly relates to a method forchecking the integrity of GPS signals for errors and for detecting GPSmultipath interference. The present invention further relates to amethod for multipath modeling and correction based on detected levels ofmultipath interference.

BACKGROUND INFORMATION

Generally, most systematic sources of error in differential GPS (DGPS),such as clock error, can be canceled or corrected to a large degreeusing conventional techniques. However, two sources of error whichcurrently cannot be readily canceled or corrected are receiver error,and errors caused by multipath interference. Receiver errors typicallyarise from a malfunction in a component of a receiver tuned to aparticular GPS satellite and have little correlation with the positionof the receiver. In contrast, multipath interference, i.e., theinterference caused by arrival of a signal from a single source at areceiver at slightly different times due to different path lengths,depends directly on the geometry of reflecting surfaces in the vicinityof the receiver in question.

More precisely, multipath interference depends on the positions of thereceiving antenna, the reflecting object(s) and the satellite geometry.Therefore, there are three distinct ways that multipath interferencechanges over time. Satellite movements change the line-of-sight vector(between the receiver and the satellite) in a slow, smooth andpredictable manner. The resulting multipath changes correlated with suchsatellite movements reflect this, in that they are low frequency, slowlychanging and “smooth”. Reflectors can be divided into two classes:moving (such as nearby vehicles) and fixed (such as the ground andnearby buildings). Multipath interference from moving objects tends tobe high frequency in nature, and weakly correlated to antenna position,whereas multipath interference caused by fixed objects is highlycorrelated to antenna (and satellite) position. In addition, when theantenna is on a mobile vehicle, its own movements can vary widely interms of speed and direction. Owing to the variability of theseconditions over even short lengths, there is usually little correlationof multipath-errors for baselines longer than a few meters.

Multipath interference therefore severely limits the achievable accuracyof DGPS. In urban areas having a high density of buildings (potentialreflectors), the accuracy of DGPS can easily degrade from about 1–2 munder multipath-free conditions to up to tens of meters. This rendersDGPS unsuitable for applications that demand high accuracy andintegrity, and in particular, forbids the use of DGPS for safetyrelevant applications.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method for checkingthe integrity of GPS measurements for a moving vehicle is provided. Themethod includes determining a first inter-vehicle distance between afirst vehicle and a second vehicle based on GPS measurements obtained atboth vehicles, independently determining a second inter-vehicle distancebased on relative motion of the first vehicle and the second vehicleobtained using INS sensors at both vehicles, and comparing the first andsecond inter-vehicle distances. The integrity of the GPS measurementsare checked if the first and second inter-vehicle distances are nearlyequivalent.

In another aspect, the present invention provides a method of detectingan error at a particular vehicle by communicating GPS data amongmultiple vehicles within a given vicinity in which test series data aregenerated at each vehicle for each pair of vehicles receiving GPSsignals from a same satellite, the test series data for each paircomprising a difference between a first inter-vehicle distance betweenthe pair of vehicle calculated based on GPS data and a secondinter-vehicle distance independently calculated based on INS sensors ineach of the pair of vehicles. Test series having values greater than athreshold are identified, indicating an error. If an error is indicated,it is then determined which of the multiple vehicles the error occurs inby comparing the test series data generated at each vehicle.

In a further aspect, there is a method of mapping GPS multipath levelsat each point in a vicinity for an entire range of satelliteconstellations. A GPS multipath error is detected at a given point inthe vicinity for a particular satellite constellation using multipleroving GPS receivers. The multipath error is recorded as a GPS multipathlevel for the particular point and satellite constellation. This processis then repeated for all other points in the vicinity and at differenttimes to capture the entire range of satellite constellations.

The present invention also provides a vehicle system for checking theintegrity of GPS measurements for a moving vehicle. The vehicle systemincludes means for receiving GPS signals and for determining a GPSpseudo range for the vehicle, means for communicating with a secondvehicle within a vicinity of the vehicle, a processor capable ofdetermining a first inter-vehicle distance between the vehicle and thesecond vehicle based on the pseudo range of the vehicle and on GPSmeasurements communicated from the second vehicle, and an INS systemincluding inertial sensors, the INS system providing informationallowing the processor to determine a relative motion of the firstvehicle. The processor determines a second inter-vehicle distance basedon the relative motion of the first vehicle and on a relative motion ofthe second vehicle communicated from the second vehicle, and comparesthe first and second inter-vehicle distances, the integrity of the GPSmeasurements being checked if the first and second inter-vehicledistances are nearly equivalent.

In another aspect, the present invention provides a system for providinga mapping of GPS multipath levels at each point in a vicinity for anentire range of satellite constellations which includes a centralinformation depository and multiple roving GPS receivers that includemeans for detecting a GPS multipath error at a given point in thevicinity for a particular satellite constellation. The multipath isrecorded as a GPS multipath level for the particular point and satelliteconstellation at the central information depository, and the detectionof multipath error is repeated for all other points in the vicinity andat different times to capture the entire range of satelliteconstellations, which are then stored as multipath levels at the centralinformation depository.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of an exemplary method for generating a testseries of data for a pair of GPS-enabled vehicles in the same vicinitywith respect to a single GPS satellite according to the presentinvention.

FIG. 2 is a schematically illustration of a method for identifying areceiver error at a particular vehicle using test series data in anexemplary scenario.

FIG. 3 is a schematic illustration of a method for identifying an INSerror at a particular vehicle using test series data for the samearrangement of vehicles and satellites as in FIG. 2.

FIG. 4 is a block diagram illustrating an exemplary driving situationwhere multipath affects GPS measurement.

FIGS. 5A and 5B show an exemplary difference in test series obtained byvehicle two between a time just before it enters a multipath region andat a time during which it is within the multipath region.

FIGS. 6A through 6D are schematic illustrations showing tracking ofchanges in multipath in an area adjacent to a building due to changes insatellite positions.

DETAILED DESCRIPTION

According to the present invention, in-vehicle navigation systems andinter-vehicle data communication are used to correct for receiver errorsand to detect multipath interference (hereinafter referred to as“multipath”). For each pair of vehicles in the same vicinity, a testseries of comparison data is generated.

FIG. 1 is a flow chart of an exemplary method for generating a testseries of data for a pair of GPS-enabled vehicles in the same vicinityaccording to the present invention. Generally, vehicles within one ortwo kilometers of each other are considered to be in the same vicinity.In an initial step 100, each vehicle takes a pseudo range measurementd₁, d₂ from the same GPS satellite. In step 110, these measurements arecommunicated from each vehicle to the other using two-way communicationor any other suitable wireless communication system. In step 115, eachvehicle computes the difference Δ₁ (=d₁−d₂) between the measurements. Asthe distance between each vehicle and the GPS satellite is far greaterthan the distance between the vehicles (the “inter-vehicle distance”),the baseline difference Δ₁ between the measured readings is equivalentto the distance vector between the vehicles projected onto the line ofsight vector to the satellite. Therefore, the difference Δ serves as ameasure of the inter-vehicle distance.

Additionally, since the GPS signals arriving at the receivers at eachrespective vehicle travel through approximately equivalent atmosphericconditions, and ephemeris errors have the same effect on both signalmeasurements, the only sources of error in the difference measurement Δ₁are multipath, receiver error, and clock error. Because clock errors canbe eliminated by using the method of single differences, multipath andreceiver error are the only sources of error that distort theinter-vehicle distance measurement that cannot be readily corrected.

As each vehicle moves, in step 120, the value of Δ also changes. Eachvehicle may be equipped with a set of independent motion sensors in aninertial navigation system (INS). These sensors may include wheel speedsensors and inertial sensors such as gyroscopes and accelerometers(collectively referred to hereinafter as INS sensors) through which therelative motion of the vehicle between points can be determined. Theutilization of inertial sensors in an INS for determining relativemotion is described in greater detail in commonly assigned andco-pending patent application Ser. No. 10/308,730, entitled “INS-basedUser Orientation and Navigation”. In step 125, each vehicle calculatesthe new inter-vehicle distance (Δ_(INS)) using the INS sensors and theoriginal baseline difference Δ₁. Simultaneously or immediatelythereafter, in step 130, a new set of GPS readings is taken and a newintervehicle distance Δ₂ is computed. In step 135, a difference (T)between the INS-based distance Δ_(INS) and the GPS-based difference Δ₂is calculated. Over short time spans, values for Δ_(INS) and for Δ₂should be close to each other, and the value of T (=Δ_(INS)−Δ₂) shouldbe close to zero.

Steps 120 to 135 are repeated numerous as both vehicles move, and aseries of comparison data points referred to as a “test series” isgathered and plotted over time. Under error-free conditions, the testseries should appear as unbiased noised with a small standard deviationof approximately one meter. If, in step 140, the test series shows asystematic bias greater than a specified threshold value of ε meters,then, in step 150, it is determined that an error exists. In this casethe error can be from three sources: receiver error in one of thevehicles, multipath error in one or both of the vehicles, and INS errorin one of the vehicles (the likelihood of a simultaneous receiver/sensorerror in both vehicles is taken to be extremely small). An error in areceiver tuned to a particular satellite can be distinguished frommultipath and INS errors by repeating the process outlined above foreach visible GPS satellite. This process isolates the particularsatellite/receiver link that is in error, e.g., it indicates that areceiver in one of the vehicles for satellite 1 is in error, while thereceivers for satellite 2 are functioning properly, but does notindicate which of the pair of vehicles the error has occurred in.Correct identification of both the type of error and the vehicle inwhich it occurs can be attained using hypothesis testing using thealready-provided GPS and INS data. Alternatively, such identificationcan be attained by sharing GPS data among multiple vehicles in the samevicinity, where “multiple” refers to a number of vehicles greater thantwo.

FIG. 2 schematically illustrates a method for identifying a receivererror at a particular vehicle using test series data in an exemplaryscenario in which four vehicles 1, 2, 3, 4 share GPS communication datawith respect to three different GPS satellites S1, S2, S3. As shown,vehicle 1 obtains a test series with respect to each of the othercommunicating vehicles for each satellite. For a first satellite S1, car1 obtains a test series T1/2 (S1) in the manner discussed above, where1/2 indicates that the test series compares vehicle 1 and vehicle 2, and(S1) denotes that the test series is taken with respect to GPSmeasurements from S1. Similarly, vehicle 1 also obtains test seriesT1/3(S1) and T1/4(S1) which are comparisons with vehicle 3 and 4,respectively. The test series are shown grouped according to thepertinent GPS satellite. Test series are also obtained with respect tosatellites S2 and S3. Each of the other vehicles obtain correspondingtest series. For example, vehicle 2 obtains test series 2/1, 2/3 and 2/4for satellites S1, S3 and S4, vehicle 3 obtains test series 3/1, 3/2 and3/4, and vehicle 4 obtains test series 4/1, 4/2, and 4/3 for satellitesS1, S2 and S3. It is generally noted that the test series T “n”/“m”(S“r”) for vehicle number “n” should be equivalent to the test series T“m”/“n” (S “r”), where n, m, and r are integers.

For the sake of illustrating the method of determining a receiver error,it is assumed that the receiver in vehicle 1 tuned to satellite S1 has asystematic error. Accordingly, all of the test series involving vehicle1 and satellite S1 show a systematic bias beyond the threshold level,indicating an error. Each of these test series are indicated withcross-hatching in FIG. 2. Since there are more than two vehicles incommunication, analysis of the test series demonstrates that while allof the test series involving vehicle 1 for satellite S1 are in error,none of the test series that do not involve vehicle 1, i.e., T2/3(S1),T2/4(S1), T3/4(S1) show any error. From this it can be deduced that thesource of the error is at vehicle 1, and not at any of the othervehicles. However, more information is required in order to distinguishwhether vehicle 1 is suffering from multipath with respect to satellite1 from a more general receiver error. If the value of the test serieschanges abruptly over a time period (such as several seconds) followingthe determination of the error, it can then be deduced that the error iscaused by multipath, and that vehicle 1 may be passing by buildings orother reflective objects.

FIG. 3 schematically illustrates a method for identifying an INS errorat a particular vehicle using test series data for the same arrangementof vehicles and satellites as in FIG. 2. As depicted, vehicle 1 has anINS error. The INS error may be caused by a malfunction in one or moreof the sensors that detect motion parameters of the vehicle. This errorcauses all estimations of relative motion of vehicle 1 to be off base,and therefore the test series concerning vehicle 1 show a systematicerror (shown with cross-hatching) unless the relative motion of vehicle1 is orthogonal to the line of sight vector from vehicle 1 to aparticular satellite. In the latter case, an error in the test serieswith respect to that satellite will not be detected. In the generalcase, in which the motion of vehicle 1 is not completely orthogonal tothe line of sight vector to any satellite, this situation differs fromthe scenario depicted in FIG. 2 depicting a receiver error in that thetest series for all three satellites S1, S2 and S3 show errors, and notonly those concerning satellite S1. In this manner, INS errors can beeasily distinguished from receiver and multipath error.

The methods for error detection and identification outlined above aremost reliable when the number of vehicles involved is high, and thenumber of errors is small. Once the number of erroneous measurementsamounts to a significant fraction of the number of actual measurementsthese methods may not work as well. However, by extending the baselinelimit for vehicle inter-communication up to tens of kilometers, thenumber of vehicles sampled for test series can be increased, albeit witha somewhat decreased level of accuracy. In an area of this size, it isgenerally possible to find a control group of vehicles which have good,error-free conditions. This group can be used as reference by the othervehicles in the area. The reference group can be changed dynamically tomaintain optimal performance, as some vehicles in the control groupencounter worse conditions, such as a region of extensive multipath, andother cars enter more benign areas.

The error isolation techniques discussed above can also be employed in amethod for multipath correction according to the present invention.Multipath can be considered a function of receiver antenna position,i.e., the position of the receiving vehicle, and satellite position:Multipath=M(l _(vh) , l _(sat1) , . . . l _(satn))where l_(vh) and l_(sat1), . . . l_(satn) represent the position of thevehicle and the position of satellites S1 to S(n), respectively. FIG. 4illustrates an exemplary driving situation where multipath affects GPSmeasurement. The area 10 shown is approximately 200 meters by 200meters, and includes vehicles 1, 2, 3 and 4 and building 20. As vehicle1 passes to the right past building 20, it enters a region 30 adjacentto the building in which GPS signals are obfuscated by reflections, andmultipath is encountered. However, only satellite S1 is at an azimuthand elevation so as to be affected, and reception from the othersatellites do not suffer from multipath degradation. As the building issquare and has an even surface, there will be a detectable difference inmultipath as vehicle 1 passes into and out of region 30.

FIGS. 5A and 5B show an exemplary difference in test series obtained byvehicle two between a time just before it enters region 30 (FIG. 5A) anda time during which it is within the region (FIG. 5B) as shown in FIG.4. In FIG. 5A, each of the test series involving vehicle 1 and satelliteS1 show a level of approximately zero meters (or receiver noise). Asshown in FIG. 5B, when vehicle 1 enters region 30, the test seriesinvolving vehicle 1 and satellite S1 jump to 10 meters (for example).After the vehicle passes through region 30, the test series reverts backto zero (not illustrated). Since, as noted above, multipath is afunction of vehicle position and satellite position, if the coordinatesof S1 are known at the time vehicle 1 enters region 30, and thegeographical coordinates of the relatively small region 30 are alsoknown, then the test series reading of 10 meters represents the level ofmultipath given these two positions.

Significantly, this detected multipath level can then be communicated tothe other vehicles 2, 3, 4 in the vicinity. The other vehicles can thenuse this information to correct the multipath level when they passthrough the same position if the position of S1 remains approximatelythe same (i.e., if only a small amount of time has passed from theinitial reading). This may be done by simply adding or subtracting, asthe case may be, 10 meters from the GPS measurements taken within region30 where the test series jump. To provide further reliability for themultipath reading, test series for other vehicles passing through theregion 30 can be used to confirm the initial reading.

The multipath detection and correction process can be extended to a fullmodeling or mapping of the multipath in a given area for a range ofsatellite constellations. An exemplary illustration of changes inmultipath in an area adjacent to a building is shown with reference toFIGS. 6A–6D. As shown in FIG. 6A, at time t₁, the configuration ofsatellite S1 and building B causes a multipath area MS1 a to formadjacent to the building. A fixed object 40 including a GPS receiver iswithin area MS1 a and is able to detect and record the level ofmultipath at this time. At a later time t₂, shown in FIG. 6B, the lineof sight to satellite S1 has moved and the area of multipath has changesfrom MS1 a to MS1 b. Since the object 40 is within MS1 b, it detects andrecords a level of multipath which may or may not be the same as thelevel detected at time t₁. At time t₃, shown in FIG. 6C, the line ofsight to S1 moves again, and a satellite S2 shifts to a position whereits signals can be detected in the area in question. In the new positionof S1, the area for multipath MS1 c has shifted so that object 40 is nolonger affected by multipath reception with respect to S1, but object 40is now within the multipath area MS2 a for satellite S2. Thus, the levelof multipath detected and recorded by object 40, which reflect themultipath with respect to S2, in effect records the total multipath atits position for the entire constellation of visible satellites S1, S2at time t₃. In FIG. 6D, showing the visible constellation at time t4, S1has moved out of the visible range, and object 40, within area MS2 b,detects and records the new multipath level with respect to S2.

Each of the recorded multipath levels at each time is recorded andstored for further use. In reality, the GPS receivers are moving(roving) vehicles, and the position at which they record multipathlevels is not fixed, but changes over time. In this manner, multipathlevels are recorded both at different times, and at different groundpositions, so that over a suitable sampling period, multipathinformation can be accumulated which describes the multipath levels in aparticular vicinity of the building B for all possible satelliteconstellations. Similarly, this process can be extended geographicallybeyond the vicinity of a single building to accumulate a multipath “map”of a region. This involves storing a large amount of data and thereforea centralized infrastructure can be used as a data repository. Thecentralized infrastructure may be equipped to broadcast this informationwireless in broadband so that each vehicle can obtain a portion of thisinformation, as need requires, to correct multipath in the area in whicheach travels. In this manner, the multipath in the vicinity of a mappedarea can be corrected for all constellations, and is thereby madevirtually multipath-free to each vehicle. As the area around thebuilding can then be considered a “benign” area, where no multipatherrors occur, the entire GPS navigation system becomes more robustagainst other errors, and vehicles in the vicinity of the building canbe used as a reference group. Additionally, each car could save the datafor roads it uses often (e.g., on the way to work) and roads in thisvicinity, and if a vehicle enters territory for which it does not havethis information, it can communicate to other vehicles local to thisarea to obtain the local multipath map.

In the foregoing description, the method and system of the presentinvention have been described with reference to a number of examplesthat are not to be considered limiting. Rather, it is to be understoodand expected that variations in the principles of the method andapparatus herein disclosed may be made by one skilled in the art, and itis intended that such modifications, changes, and/or substitutions areto be included within the scope of the present invention as set forth inthe appended claims.

1. A method of mapping GPS multipath levels at each point in a vicinityfor an entire range of satellite constellations, comprising: a)detecting a GPS multipath error at a particular point in the vicinityfor a satellite constellation using multiple GPS receivers that roveindependently with respect to one another; b) recording the multipatherror as a GPS multipath level for the particular point and thesatellite constellation; and c) repeating steps a) and b) for all otherpoints in the vicinity and at different times to capture the entirerange of satellite constellations.
 2. The method of claim 1, whereineach of the multiple roving GPS receivers generates test series data foreach pair of the roving receivers obtaining signals from a same GPSsatellite, the test series data for each pair comprising a differencebetween a first inter-vehicle distance between the pair calculated basedon GPS data, and a second inter-vehicle distance independentlycalculated based on INS sensors in each of the pair of vehicles.
 3. Asystem for providing a mapping of GPS multipath levels at each point ina vicinity for an entire range of satellite constellations, comprising:a central information depository; and multiple GPS receivers that roveindependently with respect to one another, the multiple roving GPSreceivers including means for detecting a GPS multipath error at aparticular point in the vicinity for a particular satelliteconstellation; wherein the multipath error is recorded as a GPSmultipath level for the particular point and satellite constellation atthe central information depository; and wherein the detection ofmultipath error is repeated for all other points in the vicinity and atdifferent times to capture the entire range of satellite constellations,the multipath errors being stored at the central information depository.4. The system of claim 3, wherein the central information depositoryincludes means for receiving wireless data signals, and the multipleroving GPS receivers are equipped with means for wirelessly transmittingGPS multipath errors as a data signal to the central informationdepository.